The present invention relates to a control device for an AC charging generator (alternator) for vehicular use.
A conventional device of the same general type to which the invention pertains will be described with reference to FIG. 1. In FIG. 1, reference numeral 10 designates a generator, 1 designates a three-phase winding of a stator, 2 designates a field coil, 3 designates a rectifier, 31, 32 and 33 designate first, second and third rectifying output terminals, respectively, 4 designates a storage battery, 5 designates an indicator lamp, 6 designates a keyswitch, 7 designates a control device, 71 designates a switching transistor for switching a field current, 72 designates a control transistor for controlling the transistor 71, 73 designates a Zener diode used to detect an output voltage of the generator 10 which begins to conduct when the output voltage becomes higher than a predetermined value, 74 and 75 designate first and second resistors together constituting a voltage divider circuit, and 76 designates a smoothing capacitor. The control transistor 72 and the Zener diode 73 are formed on a monolithic chip.
The conventional device of FIG. 1 operates as follows. When the keyswitch 6 is closed, the current from the storage battery 4 flows through the keyswitch 6, the indicator light 5 and the field coil 2 causing the indicator lamp 5 to light and energizing the field coil 2. Next, when the engine (not shown) starts to drive the generator 10, a voltage is generated at the first and second output terminals 31 and 32. The voltage at the first output terminal 31 is used to charge the storage battery 4. Since the same voltage is generated at the second output terminal 32 as at the first output terminal 31, the voltages at the two terminals of the indicator lamp 5 are equal, and hence the indicator lamp 5 is extinguished. When the speed of the engine further increases such that the voltage at a junction point a of the resistors 74 and 75 exceeds the predetermined value, the Zener diode 73 becomes conductive. As a result, the control transistor 72 turns on and the switching transistor 71 is turned off. Thus, the current flow through the field coil 2 is interrupted. Accordingly, the amount of current passing through the field coil 2 is decreased and the voltage generated at the output terminals is also decreased. As a result, the Zener diode 73 again becomes nonconductive. Thus, the switching transistor 71 is turned on again and the generated voltage increased. By repeating the above operations, the output of the generator 10 is maintained at a predetermined voltage.
A ripple component is present in the output of this type of generator. The magnitude of the ripple component increases as the output current increases. Since the Zener diode 73 conducts or not depending on the peak amplitude of the ripple component, an output voltage indicator, which shows a mean value, indicates a different value from the maximum value. For the purpose of reducing this difference, the smoothing capacitor 76 is provided. However, so that the Zener diode 73 will operate properly, the resistance values of the resistors 74 and 75 cannot be increased. Thus, the capacitance of the capacitor 76 must be high. Accordingly, it is difficult to fabricate the conventional circuit in hybrid IC form.